研制了一套用于4 m SiC反射镜原位检测的静压支撑系统, 以降低超大口径SiC反射镜离线检测的风险, 提高其制造效率。首先, 推导了单元刚度的解析式, 确定了其中关键因素; 然后, 对支撑单元进行抽样测试, 结合解析式预测了支撑群组中单元的工作刚度。最后, 通过密封性测试和反射镜原位检测, 验证了支撑系统的稳定性; 通过有限元模拟, 计算了系统的重力卸载面形精度。结果表明: 5个单元连组时, 单元刚度约为1.9 kN/mm, 刚度值分布在±3%误差区间; 独立单元刚度可高至15 kN/mm; 3种分组单元刚度预测值分别为1.7, 1.1和0.8 kN/mm。支撑系统空载时管路压强变化缓慢, 表明密封性良好; 用该系统支撑4 m反射镜时, 11天内高度绝对变化量小于50 μm, 相对变化量小于20 μm。54个单元刚度随机分布时, 镜面面形高阶残差(RMS)为20 nm。提出的系统基本满足原位检测的稳定性和精度要求。

A set of hydrostatic support system for a 4 m SiC mirror in-situ testing was designed to reduce the risk of ultra-large mirror off-line testing and to improve fabrication efficiency. Firstly, the analytical formula for calculating support stiffness was derived, and its main factor was found. Then, some support samples were tested, and the support stiffness of a single support unit in different groups was predicted by combining the sample results and analytical formula. Finally, the stability of the support system was evaluated by pressurization test and in-situ testing, and the mirror surface precision with its gravity offloaded by the support system was calculated by finite element simulation. Results show that the average stiffness is about 1.9 kN/mm with a relative difference among support units about 3% when 5 units are linked together, the stiffness for a single unit isolated is as high as 15 kN/mm, and 3 kinds of unit stiffness in the support system are 1.7, 1.1 and 0.8 kN/mm respectively. Moreover, pressurization test shows a slow pressure change, which indicates that the system is well sealed. When the hydrostatic system was used for a 4 m mirror, the height changes rise up and down within 50 μm in 11 days, and the relative change is less than 20 μm. When the mirror is supported on 54 units with stiffness difference by 3% randomly, the surface RMS is less than 20 nm. The proposed system meets the requirement of in-situ optical testing for precision and stability.